1. Technical Field of the Invention
The present invention relates to adapting or adjusting a data rate to maximize overall performance of a wireless local-area-network (WLAN). In particular, the present invention relates to a method for adjusting the data communication rate of an IEEE 802.11 standard WLAN connection or system.
2. Description of Related Art
The IEEE 802.11 standard for data communication provides a multi-rate capability. Transmission of data can occur at any of the IEEE 802.11 standard's specified allowable data rates. In a wireless environment, communication and channel conditions effect the maximum usable transmission rate. The multi-rate capability of the IEEE 802.11 standard is supported at the physical layer. In particular, Media Access Control (MAC) mechanisms are used to exploit the multi-rate capability and maximize the overall data and information throughput of a WLAN. The IEEE 802.11 standard does not specify how to exploit a maximum data rate for a communication channel's condition. The IEEE 802.11 standard leaves it to the individual developers to devise rate adaptation techniques that take advantage of the standard's multi-rate data communication capability.
To date, various rate adaptation schemes have been proposed and used with the IEEE 802.11 standard. The existing rate adaptation schemes can be classified into two categories, the first category being schemes that use the success and failure history of previous transmissions to aid in the selection of future data or communication rates. This first category may also be known as Auto Rate Feedback (ARF). The second category of rate adaptation schemes utilize signal measurements to aid selection of an appropriate data rate.
Providing more background about existing, first category, ARF schemes, the article Ad Kamerman et al, “Wave LAN-II: A High-Performance Wireless LAN for the Unlicensed Band,” Bell Labs Technical Journal, pp. 118-133, Summer 1997 discusses a proposal for Lucent's WaveLAN-II devices and other commercial WLAN products. The described ARF style WLAN adaptation device basically switches between 1 and 2 Mbps transmission rates based on a timing function and the number of ACK frames that are not received back by the sending device. The default transmission rate is the higher rate. The transmission rate switches to the lower rate when two consecutive ACKs are not correctly received by the sending device. At the same time that the transmission rate switches to the lower rate, a timing function begins. The transmission rate will switch back to the higher transmission rate after a predetermined amount of time or when the number of consecutively, correctly received ACKs is equal to ten. This ARF scheme is easy to program and implement without modification to the current IEEE 802.11 standard. The disadvantage of this ARF scheme is that it cannot and does not react quickly to fluctuations in channel conditions.
Another ARF style rate adaptation scheme is proposed in P. Chevillat et al, “A Dynamic Link Adaptation Algorithm for IEEE 802.11a Wireless LANs,” IEEE International Conference on Communications (ICC), pp. 1141-1146, 2003. This ARF scheme uses information, like ACKs, only available to the information sender, to estimate the channel condition. Thus, there is no feedback channel from the receiving device. This scheme uses only one failure threshold F and two success thresholds S, and S2, correspond to different channel conditions: a channel condition in a region of a higher Doppler Spread value (S1=3) or in a region of lower Doppler Spread value (S2=10).
Yet another discussion of ARF style rate adaptation schemes is discussed and evaluated in A. J. van der Vegt, “Auto Rate Fallback Algorithm for the ARF style rate adaptation scheme is very similar to the Lucent WaveLAN-II, discussed above, except for there being a different choice of fixed threshold values for determining when to switch the transmission rates to a higher or lower rate.
We now provide additional background on the second category of rate adaptation schemes, the schemes that utilize signal measurements to aid selection on an appropriate transmission data rate. In this category of rate adaptation schemes, the algorithms assume that there is additional communication between the sender and receiver regarding the communication link or channel condition. As a result, this type of rate adaptation scheme requires modifications or additions to the IEEE 802.11a standard. For example, the Receiver-Based Auto-Rate (RBAR) protocol, as discussed in Gavin Holland et al, “A Rate-Adaptive MAC Protocol for Multi-Hop Wireless Networks,” ACM MobiCom '01, pp. 236-251, 2001, requires using an RTS/CTS handshaking process, which is not part of the IEEE 802.11a standard. In this example, RBAR the receiver estimates the wireless channel condition by using a sample of the instantaneously-received signal strength from the end of the RTS reception. The RBAR protocol then feeds the transmission rate back to the sending device using the CTS packet portion of the RTS/CTS handshaking process.
Yet, another signal measuring-based rate adaptation scheme is referred to as the Opportunistic Adaptive Rate (OAR) scheme which transmits multiple data packets when the scheme believes that the channel condition is good and thereby achieves a slightly better throughput than the RBAR scheme.
In the Daji Qiao, et al, “Goodput Analysis and Link Adaptation of IEEE 802.11a Wireless LANs,” IEEE Trans. On Mobile Computing, vol. 1, no. 4, pp. 278-292, October-December 2002, article a ‘goodput’ analysis for rate adaptation in an 802.11a WLAN is presented. This article suggests a coupling of the transmission rate and the data fragment size to the channel condition estimation. A mathematic model is used with the data fragment size to aid in computing the best rate. A drawback of this rate adaptation scheme is that it assumes perfect channel knowledge and thus is of little practical use.
Another proposed rate adaptation algorithm by Javier de Prado et al, “Link Adaptation Strategy for IEEE 802.11 WLAN via Received Signal Strength Measurement,” IEEE International Conference on Communications, May 2003, proposes to use the Received Signal Strength (RSS) of received data frames, along with the number of retransmissions, to estimate the channel condition. This algorithm does not require any coordination from the receiver and thus does not require a change to the IEEE 802.11 standard. However, this scheme assumes that the signal strength of the sending device is the same as the signal strength at the receiver. In other words, the scheme assumes that a symmetrical channel condition exists. As a result that is not a practical scheme.
What is needed is a rate adaptation scheme that readily adapts to the channel conditions of a WLAN system. Such a rate adaptation scheme should be relatively easy to implement without requiring changes or modifications to the IEEE 802.11 standard.